Something else to consider is using postprocess search. This way your base search doesn't have to run every time someone chooses an option in the dropdown.
See: http://docs.splunk.com/Documentation/Splunk/latest/Viz/Savedsearches#Post-process_searches
Here's the basic idea:
dashboard loads and runs a base search that generates the full list of stats:
| STATS count BY Service
your input dropdown could access the values of 'Service' to dynamically populate your input options
you'll need to still add a static value to cover the * (all) option
your post process search, which dictates the values displayed on your dashboard, is simply:
| SEARCH Service = $dropdown_value$
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